huangyhg

anchoring column into rock
after searching for a design procedure in vain, i have come up with the following solution. please comment...
i'm wondering if anyone can offer some guidance as to how to approach a design for a 410ub54 column (that's a w410x54 in metric can/us) with a base moment of 16.7kn·m and an uplift force of 5.68kn. both are factored loads. i'm using a 440x220 base plate, with 22mm bolts for 20mm anchor bars at 145mm from the centre. (20mm bars are overkill, but i'm reducing them for threading, and the next available size (16mm) does not work when reduced for threads) what i would like to do is anchor into the volcanic bedrock below the column (rather than design a monolithic pier). the following is the procedure i have devised, and would greatly appreciate comment/criticism on:
knowns:
typical details for local volanic rock:
1000 to 2500 kg/m^3 -> assuming a value of 1200 yielding 11.77kn/m^3
1. assume a 30 deg. failure cone for the rock.
2. adopt a safety factor of 1.5 on the rock, still apply factored loadings.
3. use a non-shrink, non-metallic, pourable grout (the local nz equivalent to the familiar sika 212 is called conbextra hf)
4. base uplift on the volume of rock in the 30 deg failure cone, base the moment capacity on the action of the anchor rods as a force couple (ie: as for designing a typical base connection).
5. specify a 40mm hole in the bedrock, cleared of all debris prior to installing anchors.
thoughts:
1. i had thought about considering a 1/3 length of anchor as being unbonded at the base, ie: a portion where the bond between the pourable grout and the bars would fail.
2. given the unknowns, should i be using a sf higher than 1.5, despite the factored loads? in effect i have a sf of 1.8 on dead loads & 2.25 on live loads as it is.
my solution comes up with an embedment length of 1300mm for the anchors, and i feel quite comfortable. i am also pouring a 200x600 concrete pad for the column on the rock, to ensure a smooth & level surface. i simply would like to hear opinions on the procedure, particularly from anyone who has experience with similar situations. for the record, the volcanic bed rock is typically non-stratified in the area, and normally quite solid. in either case, i have discounted any possible help from the shear strength of the material in my solution.
thanks in advance for any help you can offer!
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
i have never worked on a project that involved anchoring into rock before, but my first step would be to obtain the needed design information from a qualified geotechnical engineer and/or a geologist. i would not be comfortable assuming a failure cone, allowable bearing pressure,or a factor of safety. once you obtain the information and have confidence in its accuracy, i would say that your logic seems reasonable.

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